What might Mark's promise to contribute to the family finances foreshadow?

Though Mark's intentions to help out with the family finances are positive, they foreshadow his career as a drug dealer, which impacts his relationship with Bryon in a negative way.
When Mrs. Douglas has to go back to the hospital, the financial pressure on the Douglas family intensifies. Though Bryon and Mark are not related to each other by blood, their close relationship means that Mark feels like Bryon is his "real" brother. Mark expresses this feeling to Bryon after Mark is injured in an attack meant for Bryon. All of these events mean that any financial pressure on the Douglas family inspires Mark to try to do his part to contribute. Unfortunately, Mark's contributions come from his dealing, which has disastrous consequences for others in Bryon's life, like his girlfriend Cathy. For example, M&M, Cathy's little brother, is hospitalized after a bad acid trip, and when Bryon finds pills in Mark's room, he realizes that Mark might be responsible for what happened to M&M.